Information about the simulation is not printed on-the-fly to `run_REXEE_log.txt`

[weitse-hsu]

The information about the simulation is not printed on-the-fly to run_REXEE_log.txt. It is only printed to the file after the simulation is completed. If the simulation is interrupted, the content of run_REXEE_log.txt will be empty. This should be corrected, presumably by modifying Logger in utils.py.

[weitse-hsu]

This can probably be addressed by simply adding self.log.flush() to Logger.write. Will try in the near future.